The invention relates to hydrogen storage alloys for use in rechargeable batteries.
A battery typically includes one or more galvanic cells (i.e., cells that produce a direct current of electricity) in a finished package. In each cell, two electrodes are separated by an electron insulator, but are joined by an ion-carrying path. The electron-carrying path of the battery is external; the path proceeds via a conductor, through a device where work is done. The ion-carrying path of the battery is internal and proceeds via an electrolyte.
The electrodes are usually composed of dissimilar metals. The electrode where the electrolyte is broken down upon the receipt of electrons is the positive electrode, also referred to as the cathode. The electrode where the metal goes into solution, releasing electrons, is called the negative electrode, or anode. The electrolyte generally is composed mainly of an ionizable salt dissolved in a solvent.
Batteries may be rechargeable; such batteries are called "storage" or "secondary" batteries. Storage batteries can be recharged by passing current through the cells in the opposite direction of current flow discharge. The chemical conditions of the battery are restored, and the cells are ready to be discharged again. Primary batteries, on the other hand, are meant to be discharged to exhaustion once, and then discarded.
An example of a rechargeable battery is a metallic oxide-hydrogen storage battery. The positive electrode this battery includes a metal oxide, such as nickel oxide; the negative electrode includes a hydrogen storage alloy; and the electrolyte includes an alkaline solution.
An example of an electrode reaction in a nickel oxide-hydrogen storage battery is as follows.
______________________________________ discharge NiOOH + H.sub.2 O + e Ni(OH).sub.2 + OH.sup.- (1) charge Negative electrode: discharge M--H + OH M + H.sub.2 O + e.sup.- (2) charge ______________________________________
In the reaction equation (2), M represents a hydrogen storage alloy.
Hydrogen storage alloys are capable of electrochemically absorbing and discharging large quantities of hydrogen. Two common types of hydrogen storage alloys are AB.sub.2 and AB.sub.5. AB.sub.2 hydrogen storage alloys are described, for example, in U.S. Pat. No. 5,277,999. AB.sub.2 hydrogen storage alloys may be based, for example, on TiNi.sub.2, and may have the general atomic structure Ni--Ti--V--Cr--Zr--x--y, where x and y are other elements.
In AB.sub.5 hydrogen storage alloys, the A and B components are present in a mole ratio of about 1:5. The A component is generally composed of a mischmetal (a mixture of rare earth elements, generally cerium (Ce), lanthanum (La), neodymium (Nd), and praseodymium (Pr)), and the B component is generally composed of nickel (Ni), along with two or more elements selected from cobalt (Co), manganese (Mn), aluminum (Al), copper (Cu), iron (Fe), zirconium (Zr), or germanium (Ge).
The subscripts of the elements forming the A component generally have a sum of 1, while the subscripts of the elements forming the B component generally have a sum of 4.75 to 5.50.
It is desirable for rechargeable batteries to have a long cycle life. Cycle life is the number of times a battery can be recharged without the discharge capacity of the battery falling below a targeted level. In metallic oxide-hydrogen storage batteries, the cycle life often is limited by the corrosion-of the hydrogen storage alloy in the negative electrode. The corrosion may result, for example, from the formation of an oxide film on the surface of the hydrogen storage alloy.